The decision, coming amid deep tensions over eastern Ukraine, provoked strong words from both sides but does not immediately affect the crucial flow of Russian gas to Europe. Ukraine has enough reserves to last until December, according to the head of its state gas company Naftogaz.

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Sergei Kupriyanov, spokesman for the Russian gas giant Gazprom, said since Ukraine missed the deadline, from now on it had to pay in advance for energy. Yet that's a nearly impossible demand for the cash-strapped nation, which is fighting an insurgency and investigating possibly billions lost to corruption under its former pro-Russian president, Viktor Yanukovych.